Improved fuel economy may be realized by deactivating some of the cylinders of a multi-cylinder engine while the remaining cylinders carry the desired load. The primary reason for the fuel economy savings is that the working cylinders operate at a higher specific loading and therefore greater manifold pressure, which results in reduced intake stroke pumping work.
Multi-cylinder engines capable of cylinder deactivation have been produced. Typically, in the case of an in-line 4 cylinder engine, two cylinders are deactivated; in the case of a V6, three cylinders (one bank) are deactivated. In both cases, cylinder deactivation is effected by disabling both intake and exhaust valves by using individual valve controllers. This causes the piston to compress and expand the trapped mass within the cylinder each revolution of the crankshaft, thereby creating a gas spring. That is, the trapped mass of gas is alternatively compressed and expanded. Because the piston merely compresses and expands the gas which is trapped in the cylinder, the friction and thermodynamic losses are relatively small and the other engine cylinders, which are actually firing, may be operated with sufficiently greater efficiency so that the overall efficiency of the engine is improved. Neglecting heat transfer and piston ring blowby losses, the work done on compression is recovered on expansion so the only work expended is the friction for sliding the piston/ring assembly in the cylinder bore and the connecting rod bearings. And, the mechanical friction of the deactivated cylinders is reduced due to significantly lower peak cylinder pressures.
Unfortunately, prior art systems which disable both intake and exhaust valves of an engine's cylinders are quite expensive and are therefore unattractive, because vehicles in which fuel economy is most important are frequently sold in the lower price range, and are therefore unable to command a price sufficient to offset the cost of the added equipment.
A different solution to cylinder deactivation is to employ a dual equal variable displacement engine. This means that an actuator mechanism is employed to phase shift the intake and exhaust camshaft(s) equally on the cylinders to be deactivated. If the valves on the deactivated cylinders are controlled by two camshafts (DOHC), one for the exhausts and one for the intakes, then the phase shifter will have to control both equally with some means of interconnection. In essence, then, they will operate as a single overhead cam for phase shifting for cylinder deactivation. Assuming a single overhead cam (SOHC) on the cylinders to be deactivated, the camshaft is retarded (or alternatively can be advanced) approximately 90 to 100 degrees from standard timing using a wide-range phase shifter. The mass that is drawn into the cylinder in the later part of the intake stroke is pushed back out during the first part of the compression stroke. The exhaust gas that is pushed out during the last part of the exhaust stroke is drawn in during the first part of the intake stroke. Thus, there is no net mass flow through the deactivated cylinders and virtual elimination of net cycle pumping work, resulting in true cylinder deactivation.
Another concern arises with both of these cylinder deactivation systems mentioned, as well as others. With a cylinder deactivation engine system, oxides of nitrogen (NOx) during part load operation may be higher than is acceptable. In a conventional engine at part load, the pressure drop between the exhaust system (typically about atmospheric pressure) and the intake manifold (much below atmospheric pressure due to throttling) induces exhaust gas recirculation (EGR) to flow from the exhaust system through a control valve in an external EGR system into the intake manifold, thus effecting the control of NOx emissions.
However, with a variable displacement engine operating with some cylinders deactivated, the firing cylinders are carrying the load that normally would be carried by the whole engine. Thus, they are operating under much higher intake manifold absolute pressures due to the lesser amount of throttling. This higher pressure reduces the inducement of the EGR gasses to flow and further, as engine load increases, will cause no EGR flow condition to occur just when NOx emissions are highest and the need for EGR the greatest.
Therefore, a cost effective and reliable means for cylinder deactivation is desirable which also addresses the concerns raised with NOx emissions.